This invention relates generally to the field of heating and cooling equipment and more particularly to an arrangement for attenuating sound waves in an air handling unit.
Large volume heating and cooling systems typically include one or more air handling units, each of which has a large fan that draws air past heating/cooling coils and directs the conditioned air into distribution ducts extending throughout the building. In most systems, it is desirable for the air handling unit to have variable volume capability so that different volumes of conditioned air can be supplied under different conditions. Due to their simplicity and relatively low cost, discharge dampers in the outlet duct section of the air handling unit are preferable to other types of variable volume devices such as adjustable inlet vanes or speed control devices.
From the standpoint of energy efficiency, fans having forward curved blades are desirable in variable volume systems because their power requirements decrease rapidly with volume decreases caused by restriction of the air flow. However, due to acoustical problems associated with forward curved fans and instability under some operating conditions, forward curved fans have not been used extensively in variable volume air handling units.
The cause of oscillations leading to acoustical problems and instability can best be understood by referring to FIG. 3 of the accompanying drawings which is a graph of the pressure versus flow characteristic of a forward curved fan. The system normally operates at or near point A on the fan curve which is to the right of the fan curve maximum. However, when the discharge duct is restricted by partial closing of the dampers, the operating point can be shifted to the left of the fan curved peak to point B. Then, at frequencies where the acoustical resistance of the system (pressure divided by flow) seen by the fan is lower than the fan resistance described by the slope of the fan curve at point B, instabilities occur since the fan provides more pressure than is absorbed by the system as a whole.
When the dampers are closed or partially closed, the resulting oscillating variation in the operating point seen by the fan creates an acoustical wave which travels downstream from the fan. When the acoustical discontinuity represented by the partially closed dampers is encountered by the acoustical wave, it is reflected back toward the fan. The reflected wave arrives back at the fan after a time delay T=2L/C, where L is the distance between the fan and dampers and C is the speed of sound in air. If the round trip time delay T is such that the pressure of the reflected acoustical wave is opposite to the pressure of the transmitted wave, the pressures tend to cancel, and the combined pressure is lower than the pressure normally seen by the fan wheel. The fan wheel then senses a lower system resistance than is described by the system characteristic at point B on the fan curve. This effect occurs when the oscillation frequency f is approximately equal to C/4L which represents a condition where the dampers are one quarter wave length downstream from the fan blades.
In cases where the amplitude of the reflected wave is nearly equal to that of the transmitted wave, there is nearly complete pressure cancellation and the acoustical resistance seen by the fan at the frequency f=C/4L is nearly zero. Under these conditions, the slope of the acoustical resistance line is more horizontal than the slope of the fan curve at point B, and the fan provides more pressure than the system absorbs. The amplitude of the oscillation then increases to a potentially large value, and an undesirable surge effect occurs. Such an unstable condition is represented by the broken line in FIG. 3 labeled "Unstable Acoustical Resistance". There is no instability present when the slope of the acoustical resistance line is steeper than that of the fan curve, as indicated by the broken line labeled "Stable Acoustical Resistance" in FIG. 3.
Undesirable oscillation can be lessened by attenuating the sound waves so that the amplitude of the reflected wave is reduced substantially by the time it reaches the fan wheel. Then, there is little pressure cancellation and the acoustical resistance seen by the fan is only reduced by a small amount. The refected waves are normally reduced in their amplitudes by interposing sound absorbing material in the duct between the discharge of the fan and the dampers.
It would seem logical that maximum attenuation could be achieved by providing a long distance between the fan and damper in order to provide maximum attenuation length. However, this is not the case in actual practice because the frequency at which oscillation occurs (f=C/4L) decreases with increasing distance between the fans and dampers, and lower frequencies are more difficult to attenuate. In typical sound absorbing media, the attenuation in the frequency range of interest decreases at a greater than linear rate with decreasing frequency. Since the critical frequency (f=C/4L) varies linearly with the distance between the fan and dampers, the attenution per unit length decreases rapidly within increasing distance between fan and dampers. As a consequence, we have found that it is generally undesirable from an acoustical standpoint to locate the dampers a considerable distance downstream of the fan.
The primary goal of the present invention is to achieve stable acoustical characteristics in an air handling unit employing a forwardly curved fan. In accordance with the invention, the distance between the fan and dampers is short enough to result in a relatively high critical frequency which can be easily attenuated by conventional acoustical materials. Preferably, the average distance between the fan blades and dampers is about 30 inches, and the duct work in this area is lined with acoustical panels. The closed damper resonant frequency is then relatively high and is readily attenuated by the acoustical panels. The overall result is that acoustical instabilities and surge effects are avoided in a simple and effective manner without requiring long duct work or other extensive additions to or modifications of the basic air handling unit.